Prevention of Brca1-Mediated Mammary Tumorigenesis in Mice by a Progesterone Antagonist

Decreased expression of Yes-associated protein is associated with outcome in the luminal A breast cancer subgroup and with an impaired tamoxifen response

BACKGROUND

Yes-associated protein (YAP1) is frequently reported to function as an oncogene in many types of cancer, but in breast cancer results remain controversial. We set out to clarify the role of YAP1 in breast cancer by examining gene and protein expression in subgroups of patient material and by downregulating YAP1 in vitro and studying its role in response to the widely used anti-estrogen tamoxifen.

METHODS

YAP1 protein intensity was scored as absent, weak, intermediate or strong in two primary breast cancer cohorts (n = 144 and n = 564) and mRNA expression of YAP1 was evaluated in a gene expression dataset (n = 1107). Recurrence-free survival was analysed using the log-rank test and Cox multivariate analysis was used to test for independence. WST-1 assay was employed to measure cell viability and a luciferase ERE (estrogen responsive element) construct was used to study the effect of tamoxifen, following downregulation of YAP1 using siRNAs.

RESULTS

In the ER+ (Estrogen Receptor α positive) subgroup of the randomised cohort, YAP1 expression was inversely correlated to histological grade and proliferation (p = 0.001 and p = 0.016, respectively) whereas in the ER- (Estrogen Receptor α negative) subgroup YAP1 expression correlated positively to proliferation (p = 0.005). Notably, low YAP1 mRNA was independently associated with decreased recurrence-free survival in the gene expression dataset, specifically for the luminal A subgroup (p < 0.001) which includes low proliferating tumours of lower grade, usually associated with a good prognosis. This subgroup specificity led us to hypothesize that YAP1 may be important for response to endocrine therapies, such as tamoxifen, extensively used for luminal A breast cancers. In a tamoxifen randomised patient material, absent YAP1 protein expression was associated with impaired tamoxifen response which was significant upon interaction analysis (p = 0.042). YAP1 downregulation resulted in increased progesterone receptor (PgR) expression and a delayed and weaker tamoxifen in support of the clinical data.

CONCLUSIONS

Decreased YAP1 expression is an independent prognostic factor for recurrence in the less aggressive luminal A breast cancer subgroup, likely due to the decreased tamoxifen sensitivity conferred by YAP1 downregulation.

Molecular determinants of context-dependent progesterone receptor action in breast cancer

The ovarian steroid hormone, progesterone, and its nuclear receptor, the progesterone receptor, are implicated in the progression of breast cancer. Clinical trial data on the effects of hormone replacement therapy underscore the importance of understanding how progestins influence breast cancer growth. The progesterone receptor regulation of distinct target genes is mediated by complex interactions between the progesterone receptor and other regulatory factors that determine the context-dependent transcriptional action of the progesterone receptor. These interactions often lead to post-translational modifications to the progesterone receptor that can dramatically alter receptor function, both in the normal mammary gland and in breast cancer. This review highlights the molecular components that regulate progesterone receptor transcriptional action and describes how a better understanding of the complex interactions between the progesterone receptor and other regulatory factors may be critical to enhancing the clinical efficacy of anti-progestins for use in the treatment of breast cancer.

The potential of selective progesterone receptor modulators for the treatment of uterine fibroids

In addition to the estrogen receptor, the progesterone receptor plays an important role in the growth of uterine fibroids. Several selective progesterone receptor modulators (SPRMs) have been evaluated for medical treatment of uterine fibroids and, because of safety issues, some molecules were stopped during clinical development. However, in 2012, ulipristal acetate received the approval for a pre-surgical treatment of uterine fibroids. Clinical trials with ulipristal acetate for long-term medical treatment of uterine fibroids are ongoing. This review article describes the action of SPRMs at the progesterone receptor level and the mechanism of action on the fibroid tissue. A review of the published clinical trials is performed, including the current evidence of efficacy on uterine fibroid symptom management, size reduction and tolerability. The therapeutic potential of SPRMs for uterine fibroids is discussed.

Mouse models of BRCA1 and their application to breast cancer research

Germline mutations of human breast cancer-associated gene 1 (BRCA1) predispose women to breast and ovarian cancers. In mice, over 20 distinct mutations, including null, hypomorphic, isoform, conditional, and point mutations, have been created to study functions of Brca1 in mammary development and tumorigenesis. Analyses using these mutant mice have yielded an enormous amount of information that greatly facilitates our understanding of the gender- and tissue-specific tumor suppressor functions of BRCA1, as well as enriches our insights into applying these preclinical models of disease to breast cancer research. Here, we review features of these mutant mice and their applications to cancer prevention and therapeutic treatment.

The sex hormone system in carriers of BRCA1/2 mutations: a case-control study

BACKGROUND

Penetrance for breast cancer, ovarian cancer, or both in carriers of BRCA1/BRCA2 mutations is disproportionately high. Sex hormone dysregulation and altered end-organ hormone sensitivity might explain this organ-specific penetrance. We sought to identify differences in hormone regulation between carriers of BRCA1/2 and women who are negative for BRCA1/2 mutations.

METHODS

We assessed endometrial thickness for each menstrual cycle day (as an index of hormone regulation) in 393 scans from 228 women in the UK Familial Ovarian Cancer Screening Study (UK FOCSS) known to carry either mutation and 1573 scans from 754 women known to be negative for the mutations. To quantify differences in endometrial thickness we focused on days 10-14 and days 21-26, and calculated the area under the curve. We then compared serum oestradiol and progesterone titres during these days of the menstrual cycle in the same groups. Follicular and luteal oestradiol and progesterone serum titres were grouped into quartiles and odds ratios were calculated with logistic regression.

FINDINGS

Follicular phase endometrial thickness of carriers of the mutations adjusted for age and day of the menstrual cycle was higher (odds ratio [OR] 1·11, 95% CI 1·03-1·20; p=0·0063) and luteal phase endometrial thickness lower (0·90, 0·83-0·98; p=0·027) than for women negative for the mutations. Median luteal phase titres of progesterone were 121% higher (p=0·00037) in carriers than in women negative for the mutations, and for oestradiol were 33% higher (p=0·007)-ie, 59% of carriers had concentrations of serum progesterone that would have been in the top quartile of concentrations in the control group (OR 8·0, 95% CI 2·1-52·57; p=0·008).

INTERPRETATION

Carriers of BRCA1/BRCA2 mutations are exposed to higher titres of oestradiol and progesterone-known risk-factors for breast cancer. Higher titres of oestradiol in carriers are compatible with this hormone having a role in ovarian carcinogenesis in such women. Our findings could not be explained by differential contraceptive pill use.

Synthesis and antiprogestational properties of novel 17-fluorinated steroids

Progesterone receptor (PR) plays a key role in reproductive functions, and compounds that inhibit progesterone action (antiprogestins) have potential use in the treatment of estrogen- and progesterone-dependent diseases, including uterine leiomyomas and breast cancer. In the present study, we chemically synthesized novel 17-fluorinated steroids and evaluated the cytotoxicity profiles of these compounds in T47D breast cancer cells compared to the activity of known antiprogestins, including ZK230 211, RU-486, CDB2914, CDB4124 and ORG33628. We analyzed in vitro receptor-binding assays and PR-transactivation assays to establish the antiprogestational activity of these molecules. The representative antiprogestin EC304 was found to inhibit in vitro tumorigenicity in a dose-dependent fashion in T47D cells by a colony formation assay at 1 and 10nM concentrations. The potent in vivo antiprogestational activity of EC304 was also demonstrated in an antinidation assay for the interruption of early pregnancy in rats. The strong antiprogestational activity and absence of antiglucocorticoid activity in EC compounds may demonstrate their utility in the treatment of leiomyoma, endometriosis and breast cancer.

A novel BRCA1 mutation in a patient with breast and ovarian cancer: A case report

Germline mutations in the human breast cancer genes BRCA1 and BRCA2 account for a substantial proportion of familial, early-onset breast and ovarian cancers. The present study reports a novel disease-causing BRCA1 mutation, nucleotide 3020insCT/c.2901insCT, in a 55-year-old Spanish female with breast and ovarian cancer. This frameshift mutation creates a premature stop codon at amino acid 1000, leading to a truncated BRCA1 protein. To the best of our knowledge, this mutation has not been previously described in the Breast Cancer Information Core (BIC) database or the published literature.

Research resource: progesterone receptor targetome underlying mammary gland branching morphogenesis

Progesterone (P4)-activated progesterone receptors (PRs) play an essential role in driving pregnancy-associated mammary ductal side-branching morphogenesis and alveologenesis. However, the global cistromic and transcriptome responses that are required to elicit P4-dependent branching morphogenesis have not been elucidated. By combining chromatin immunoprecipitation followed by deep sequencing to identify genome-wide PR-binding sites in PR-positive luminal epithelial cells with global gene expression signatures acutely regulated by PRs in the mammary gland, we have identified a mammary epithelial PR targetome associated with active P4-dependent branching morphogenesis in vivo. We demonstrate that P4-induced side-branching is initiated by epithelial cell rearrangement into a multilayered epithelium that sprouts laterally from quiescent ducts via a mechanism requiring P4-dependent activation of Rac-GTPase signaling. We identify effectors of Rac-GTPases as direct transcriptional targets of PRs, and we demonstrate that disruption of the P4-activated Rac-GTPase signaling axis is sufficient to eliminate P4-dependent side-branching. Our data reveal that the molecular mediators of P4-dependent ductal side-branching overlap with those implicated in breast cancer.

[Interest of selective progesterone receptor modulators in endometriosis]

The SPRM (selective progesterone receptor modulators) are agonists and/or antagonists of progesterone receptor. They are responsible for anovulation, amenorrhea and a lower prostaglandin levels, which leads to an improvement in pain and regression of lesions in endometriosis. On the endometrium, a particular aspect, the progesterone receptor modulator-associated endometrial changes (PAEC), raises additional studies to verify its harmlessness. However, due to the lack of hypoestrogenism and metabolic effects with these drugs, it is very likely that the SPRM will in the near future an important place in the treatment of endometriosis.

Progesterone receptor A stability is mediated by glycogen synthase kinase-3β in the Brca1-deficient mammary gland

Germ line mutations of the BRCA1 gene increase the risk of breast and ovarian cancer, but the basis of this tissue-specific tumor predisposition is not fully understood. Previously, we reported that the progesterone receptors are stabilized in Brca1-deficient mammary epithelial cells, and treating with anti-progesterone delays mammary tumorigenesis in Brca1/p53 conditional knock-out mice, suggesting that the progesterone has a critical role in breast carcinogenesis. To further explore how the stability of progesterone receptor is modulated, here, we have found that glycogen synthase kinase (GSK)-3β phosphorylation of progesterone receptor-A (PR-A) facilitates its ubiquitination. GSK-3β-mediated phosphorylation of serine 390 in PR-A regulates its subsequent ubiquitination and protein stability. Expression of PR-A(S390A) mutant in the human breast epithelial cells, MCF-10A, results in enhanced proliferation and formation of aberrant acini structure in the three-dimensional culture. Consistently, reduction of phosphorylation of serine 390 of PR-A and GSK-3β activity is observed in the Brca1-deficient mammary gland. Taken together, these results provide important aspects of tissue specificity of BRCA1-mediated suppression of breast carcinogenesis.

Epimorphin is a novel regulator of the progesterone receptor isoform-a

Epimorphin/syntaxin-2 is a membrane-tethered protein localized extracellularly (Epim) and intracellularly (Stx-2). The extracellular form Epim stimulates morphogenic processes in a range of tissues, including in murine mammary glands where its overexpression in luminal epithelial cells is sufficient to drive hyperplasia and neoplasia. We analyzed WAP-Epim transgenic mice to gain insight into how Epim promotes malignancy. Ectopic overexpression of Epim during postnatal mammary gland development led to early side-branching onset, precocious bud formation, and increased proliferation of mammary epithelial cells. Conversely, peptide-based inhibition of Epim function reduced side branching. Because increased side branching and hyperplasia occurs similarly in mice upon overexpression of the progesterone receptor isoform-a (Pgr-a), we investigated whether Epim exhibits these phenotypes through Pgr modulation. Epim overexpression indeed led to a steep upregulation of both total Pgr mRNA and Pgr-a protein levels. Notably, the Pgr antagonist RU486 abrogated Epim-induced ductal side branching, mammary epithelial cell proliferation, and bud formation. Evaluation of Epim signaling in a three-dimensional ex vivo culture system showed that its action was dependent on binding to its extracellular receptor, integrin-αV, and on matrix metalloproteinase 3 activity downstream of Pgr-a. These findings elucidate a hitherto unknown transcriptional regulator of Pgr-a, and shed light on how overexpression of Epim leads to malignancy.

A new strategy for selective targeting of progesterone receptor with passive antagonists

Currently available progesterone (P4) receptor (PR) antagonists, such as mifepristone (RU486), lack specificity and display partial agonist properties, leading to potential drawbacks in their clinical use. Recent x-ray crystallographic studies have identified key contacts involved in the binding of agonists and antagonists with PR opening the way for a new rational strategy for inactivating PR. We report here the synthesis and characterization of a novel class of PR antagonists (APRn) designed from such studies. The lead molecule, the homosteroid APR19, displays in vivo endometrial anti-P4 activity. APR19 inhibits P4-induced PR recruitment and transactivation from synthetic and endogenous gene promoters. Importantly, it exhibits high PR selectivity with respect to other steroid hormone receptors and is devoid of any partial agonist activity on PR target gene transcription. Two-hybrid and immunostaining experiments reveal that APR19-bound PR is unable to interact with either steroid receptor coactivators 1 and 2 (SRC1 and SCR2) or nuclear receptor corepressor (NcoR) and silencing mediator of retinoid acid and thyroid hormone receptor (SMRT), in contrast to RU486-PR complexes. APR19 also inhibits agonist-induced phosphorylation of serine 294 regulating PR transcriptional activity and turnover kinetics. In silico docking studies based on the crystal structure of the PR ligand-binding domain show that, in contrast to P4, APR19 does not establish stabilizing hydrogen bonds with the ligand-binding cavity, resulting in an unstable ligand-receptor complex. Altogether, these properties highly distinguish APR19 from RU486 and likely its derivatives, suggesting that it belongs to a new class of pure antiprogestins that inactivate PR by a passive mechanism. These specific PR antagonists open new perspectives for long-term hormonal therapy.

A portrayal of E3 ubiquitin ligases and deubiquitylases in cancer

E3 ubiquitin ligases and deubiquitylating enzymes (DUBs) are the key components of ubiquitin proteasome system which plays a critical role in cellular protein homeostasis. Any shortcoming in their biological roles can lead to various diseases including cancer. The dynamic interplay between ubiquitylation and deubiquitylation determines the level and activity of several proteins including p53, which is crucial for cellular stress response and tumor suppression pathways. In this review, we describe the different types of E3 ubiquitin ligases including those targeting tumor suppressor p53, SCF ligases and RING type ligases and accentuate on biological functions of few important E3 ligases in the cellular regulatory networks. Tumor suppressor p53 level is tightly regulated by multiple E3 ligases including Mdm2, COP1, Pirh2, etc. SCF ubiquitin ligase complexes are key regulators of cell cycle and signal transduction. BRCA1 and VHL RING type ligases function as tumor suppressors and play an important role in DNA repair and hypoxia response respectively. Further, we discuss the biological consequences of deregulation of the E3 ligases and the implications for cancer development. We also describe deubiquitylases which reverse the process of ubiquitylation and regulate diverse cellular pathways including metabolism, cell cycle control and chromatin remodelling. As the E3 ubiquitin ligases and DUBs work in a substrate specific manner, an improved understanding of them can lead to better therapeutics for cancer.

Progesterone receptor isoforms PRA and PRB differentially contribute to breast cancer cell migration through interaction with focal adhesion kinase complexes

Conditionally expressed progesterone receptor isoforms PRA and PRB enhance breast cancer cell migration through interaction with focal adhesion kinase (FAK) and differential regulation of FAK phosphorylation and turnover. PRB-stimulated migration is reduced by progestins, which is prevented by PR antagonists or agonist-bound PRA.

Progesterone receptor (PR) and progestins affect mammary tumorigenesis; however, the relative contributions of PR isoforms A and B (PRA and PRB, respectively) in cancer cell migration remains elusive. By using a bi-inducible MDA-MB-231 breast cancer cell line expressing PRA and/or PRB, we analyzed the effect of conditional PR isoform expression. Surprisingly, unliganded PRB but not PRA strongly enhanced cell migration as compared with PR(–) cells. 17,21-Dimethyl-19-norpregna-4,9-dien-3,20-dione (R5020) progestin limited this effect and was counteracted by the antagonist 11β-(4-dimethyl­amino)­phenyl-17β-hydroxy-17-(1-propynyl)­estra-4,9-dien-3-one (RU486). Of importance, PRA coexpression potentiated PRB-mediated migration, whereas PRA alone was ineffective. PR isoforms differentially regulated expressions of major players of cell migration, such as urokinase plasminogen activator (uPA), its inhibitor plasminogen activator inhibitor type 1, uPA receptor (uPAR), and β1-integrin, which affect focal adhesion kinase (FAK) signaling. Moreover, unliganded PRB but not PRA enhanced FAK Tyr397 phosphorylation and colocalized with activated FAK in cell protrusions. Because PRB, as well as PRA, coimmunoprecipitated with FAK, both isoforms can interact with FAK complexes, depending on their respective nucleocytoplasmic trafficking. In addition, FAK degradation was coupled to R5020-dependent turnovers of PRA and PRB. Such an effect of PRB/PRA expression on FAK signaling might thus affect adhesion/motility, underscoring the implication of PR isoforms in breast cancer invasiveness and metastatic evolution with underlying therapeutic outcomes.

Progesterone action in endometrial cancer, endometriosis, uterine fibroids, and breast cancer

Progesterone receptor (PR) mediates the actions of the ovarian steroid progesterone, which together with estradiol regulates gonadotropin secretion, prepares the endometrium for implantation, maintains pregnancy, and differentiates breast tissue. Separation of estrogen and progesterone actions in hormone-responsive tissues remains a challenge. Pathologies of the uterus and breast, including endometrial cancer, endometriosis, uterine fibroids, and breast cancer, are highly associated with estrogen, considered to be the mitogenic factor. Emerging evidence supports distinct roles of progesterone and its influence on the pathogenesis of these diseases. Progesterone antagonizes estrogen-driven growth in the endometrium, and insufficient progesterone action strikingly increases the risk of endometrial cancer. In endometriosis, eutopic and ectopic tissues do not respond sufficiently to progesterone and are considered to be progesterone-resistant, which contributes to proliferation and survival. In uterine fibroids, progesterone promotes growth by increasing proliferation, cellular hypertrophy, and deposition of extracellular matrix. In normal mammary tissue and breast cancer, progesterone is pro-proliferative and carcinogenic. A key difference between these tissues that could explain the diverse effects of progesterone is the paracrine interactions of PR-expressing stroma and epithelium. Normal endometrium is a mucosa containing large quantities of distinct stromal cells with abundant PR, which influences epithelial cell proliferation and differentiation and protects against carcinogenic transformation. In contrast, the primary target cells of progesterone in the breast and fibroids are the mammary epithelial cells and the leiomyoma cells, which lack specifically organized stromal components with significant PR expression. This review provides a unifying perspective for the diverse effects of progesterone across human tissues and diseases.

A novel small molecule RAD51 inactivator overcomes imatinib-resistance in chronic myeloid leukaemia

RAD51 recombinase activity plays a critical role for cancer cell proliferation and survival, and often contributes to drug-resistance. Abnormally elevated RAD51 function and hyperactive homologous recombination (HR) rates have been found in a panel of cancers, including breast cancer and chronic myeloid leukaemia (CML). Directly targeting RAD51 and attenuating the deregulated RAD51 activity has therefore been proposed as an alternative and supplementary strategy for cancer treatment. Here we show that a newly identified small molecule, IBR2, disrupts RAD51 multimerization, accelerates proteasome-mediated RAD51 protein degradation, reduces ionizing radiation-induced RAD51 foci formation, impairs HR, inhibits cancer cell growth and induces apoptosis. In a murine imatinib-resistant CML model bearing the T315I Bcr-abl mutation, IBR2, but not imatinib, significantly prolonged animal survival. Moreover, IBR2 effectively inhibits the proliferation of CD34(+) progenitor cells from CML patients resistant to known BCR-ABL inhibitors. Therefore, small molecule inhibitors of RAD51 may suggest a novel class of broad-spectrum therapeutics for difficult-to-treat cancers.

The hyperplastic phenotype in PR-A and PR-B transgenic mice: lessons on the role of estrogen and progesterone receptors in the mouse mammary gland and breast cancer

Progesterone receptor (PR) belongs to the superfamily of steroid receptors and mediates the action of progesterone in its target tissues. In the mammary gland, in particular, PR expression is restricted to the luminal epithelial cell compartment. The generation of estrogen receptor-α (ER) and PR knockout mice allowed the specific characterization of the roles of each of these in mammary gland development: ER is critical for ductal morphogenesis, whereas PR has a key role in lobuloalveolar differentiation. To further study the role PR isoforms have in mammary gland biology, transgenic mice overexpressing either the "A" (PR-A) or the "B" (PR-B) isoforms of PR were generated. Overexpression of the A isoform of PR led to increased side branching, multilayered ducts, loss of basement membrane integrity, and alterations in matrix metalloproteinase activation in the mammary gland. Moreover, levels of TGFβ1 and p21 were diminished and those of cyclin D1 increased. Interestingly, the phenotype was counteracted by antiestrogens, suggesting that ER is essential for the manifestation of the hyperplasias. Mice overexpressing the B isoform of PR had limited ductal growth but retained the ability to differentiate during pregnancy. Levels of latent and active TGFβ1 were increased compared to PR-A transgenics. The phenotypes of these transgenic mice are further discussed in the context of the impact of progesterone on mammary stem cells and breast cancer. We conclude that an adequate balance between the A and B isoforms of PR is critical for tissue homeostasis. Future work to further understand the biology of PR in breast biology will hopefully lead to new and effective preventive and therapeutic alternatives for patients.

BRCA1--conductor of the breast stem cell orchestra: the role of BRCA1 in mammary gland development and identification of cell of origin of BRCA1 mutant breast cancer

Breast cancer treatment has been increasingly successful over the last 20 years due in large part to targeted therapies directed against different subtypes. However, basal-like breast cancers still represent a considerable challenge to clinicians and scientists alike since the pathogenesis underlying the disease and the target cell for transformation of this subtype is still undetermined. The considerable similarities between basal-like and BRCA1 mutant breast cancers led to the hypothesis that these cancers arise from transformation of a basal cell within the normal breast epithelium through BRCA1 dysfunction. Recently, however, a number of studies have called this hypothesis into question. This review summarises the initial findings which implicated the basal cell as the cell of origin of BRCA1 related basal-like breast cancers, as well as the more recent data which identifies the luminal progenitor cells as the likely target of transformation. We compare a number of key studies in this area and identify the differences that could explain some of the contradictory findings. In addition, we highlight the role of BRCA1 in breast cell differentiation and lineage determination by reviewing recent findings in the field and our own observations suggesting a role for BRCA1 in stem cell regulation through activation of the p63 and Notch pathways. We hope that through an increased understanding of the BRCA1 role in breast differentiation and the identification of the cell(s) of origin we can improve treatment options for both BRCA1 mutant and basal-like breast cancer subgroups.

BRCA1 targets G2/M cell cycle proteins for ubiquitination and proteasomal degradation

The BRCA1 tumor suppressor protein heterodimerizes with its partner protein, BARD1, via the RING domain present in both proteins. The heterodimer contains an E3 ubiquitin ligase activity and participates in multiple cellular functions such as cell cycle control, DNA repair and regulation of gene transcription, collectively aimed at maintaining genomic stability and tumor suppression. Yet, the precise role of BRCA1 E3 ligase in these cellular functions is poorly understood. We present data showing that BRCA1 ubiquitinates G2/M cell cycle proteins, cyclin B and Cdc25C, leading to their accelerated degradation via a mechanism that is independent of APC/C. BRCA1-dependent degradation of cyclin B and Cdc25C is reversed by proteasome inhibitors and is enhanced following DNA damage, which may represent a possible mechanism to prevent cyclin B and Cdc25C accumulation, a requirement for mitotic entry. Our data provide mechanistic insight into how BRCA1 E3 ligase activity regulates the G2/M cell cycle checkpoint and, thus, contributes to maintenance of genomic stability.

Canid progesterone receptors lack activation function 3 domain-dependent activity

Progesterone regulates multiple behavioral, physiological, and pathological aspects of female reproductive biology through its two progesterone receptors (PRs), PR-B and the truncated PR-A. PR-B is necessary for mammary gland development in mice and, compared with PR-A, is overall a stronger transactivator of target genes due to an additional activation function 3 (AF3) domain. In dogs, known for their high sensitivity to progesterone-induced mammary cancer, the PR-B function was studied. Canine PR (cPR)-B appeared to contain multiple mutations within AF3 core sequence motifs and lacks N-terminal ligand-independent posttranslational modifications. Consequently, cPR-B has a weak transactivation potential on progesterone-responsive mouse mammary tumor virus-luc and progesterone response element 2-luc reporters transiently transfected in hamster, human, or canine cells and also on known target genes FKBP5 and SGK in doxycycline-inducible, stable transfected cPR-B in canine mammary cells. The cPR-B function was restored to the level of human PR-B by the replacement of canine AF3 domain with the human one. The lack of AF3 domain-dependent transcriptional activity was unique for canids (gray wolf, red fox, and raccoon dog) and not present in closely related caniform species (brown bear, gray seal, and domestic ferret). Despite the limited transactivation potential, canids develop normal mammary glands and frequently mammary tumors. Therefore, these results question the role of PR-B in breast cancer development and may explain unique features of canid reproduction.

Differential regulation of breast cancer-associated genes by progesterone receptor isoforms PRA and PRB in a new bi-inducible breast cancer cell line

Progesterone receptor isoforms (PRA and PRB) are expressed at equal levels in normal mammary cells. However, alteration in PRA/PRB expression is often observed in aggressive breast cancer suggesting differential contribution of PR isoforms in carcinogenesis. The mechanisms underlying such processes remain to be established mainly due to paucity of appropriate cellular models. To investigate the role of PR isoforms and the impact of imbalanced PRA/PRB ratio in transcriptional regulation, we have generated an original human breast cancer cell line conditionally expressing PRA and/or PRB in dose-dependence of non-steroid inducers. We first focused on PR-dependent transcriptional regulation of the paracrine growth factor gene amphiregulin (AREG) playing important role in cancer. Interestingly, unliganded PRA increases AREG expression, independently of estrogen receptor, yet inhibitable by antiprogestins. We show that functional outcome of epidermal growth factor (EGF) on such regulation is highly dependent on PRA/PRB ratio. Using this valuable model, genome-wide transcriptomic studies allowed us to determine the differential effects of PRA and PRB as a function of hormonal status. We identified a large number of novel PR-regulated genes notably implicated in breast cancer and metastasis and demonstrated that imbalanced PRA/PRB ratio strongly impact their expression predicting poor outcome in breast cancer. In sum, our unique cell-based system strongly suggests that PRA/PRB ratio is a critical determinant of PR target gene selectivity and responses to hormonal/growth factor stimuli. These findings provide molecular support for the aggressive phenotype of breast cancers with impaired expression of PRA or PRB.

The immninent dawn of SPRMs in obstetrics and gynecology

Selective progesterone receptor modulators (SPRMs) have been developed since the late 70s when mifepristone was first described. They act through nuclear progesterone receptors and can have agonist or mixed agonist antagonist actions depending on the cell and tissue. Mifepristone has unique major antagonist properties allowing its use for pregnancy termination. Ulipristal acetate has been marketed in 2009 for emergency contraception and has been recently approved for preoperative myoma treatment. Further perspectives for SPRMs use include long term estrogen free contraception, endometriosis treatment. However long term applications will be possible only after confirmation of endometrial safety.

Ulipristal acetate does not impact human normal breast tissue

BACKGROUND

Antiprogestins are of growing interest for the development of new treatments in the gynecological field. Ulipristal acetate (UPA) is a progesterone receptor (PR) modulator considered for long-term administration in contraception and is currently being registered for the treatment of uterine fibroids. In light of the influences of hormonal dysfunction in breast pathologies, the secondary consequences of chronic UPA therapy need to be established. The aim of this study was to determine UPA actions mediated by PR and glucocorticoid receptor (GR) in normal and transformed breast.

METHODS

UPA, progesterone (P) and dexamethasone (DEX) effects were observed on PR and GR responsive genes and on proliferation and apoptosis of normal human breast epithelial (HBE) and breast cancer cells. Human normal breast tissue samples were xenografted in athymic mice and treated with estradiol (E2), or E2 + P, or E2 + P + UPA.

RESULTS

Analysis of PR and GR reporter gene transactivation and their respective endogenous target genes indicated that UPA exerted anti-progestational and anti-glucocorticoid activity in both types of cells with a more pronounced effect in cancer cells. When combined with P or DEX, UPA limits the proliferation of HBE cells but increases growth in breast cancer cell lines. UPA administration had no impact on the mitotic index on xenografted human breast tissue exposed to gonadal hormones at similar concentrations to those present in normal women.

CONCLUSIONS

Although further clinical trials are required to confirm that the results from our experimental models can be extrapolated to women treated with UPA, they suggest that such treatment would not be deleterious to normal breast tissue at least for a cycle (28 days) of continuous administration.

The biology of progesterone receptor in the normal mammary gland and in breast cancer

This paper reviews work on progesterone and the progesterone receptor (PR) in the mouse mammary gland that has been used extensively as an experimental model. Studies have led to the concept that progesterone controls proliferation and morphogenesis of the luminal epithelium in a tightly orchestrated manner at distinct stages of development by paracrine signaling pathways, including receptor activator of nuclear factor κB ligand (RANKL) as a major paracrine factor. Progesterone also drives expansion of stem cells by paracrine signals to generate progenitors required for alveologenesis. During mid-to-late pregnancy, progesterone has another role to suppress secretory activation until parturition mediated in part by crosstalk between PR and prolactin/Stat5 signaling to inhibit induction of milk protein gene expression, and by inhibiting tight junction closure. In models of hormone-dependent mouse mammary tumors, the progesterone/PR signaling axis enhances pre-neoplastic progression by a switch from a paracrine to an autocrine mode of proliferation and dysregulation of the RANKL signaling pathway. Limited experiments with normal human breast show that progesterone/PR signaling also stimulates epithelial cell proliferation by a paracrine mechanism; however, the signaling pathways and whether RANKL is a major mediator remains unknown. Work with human breast cancer cell lines, patient tumor samples and clinical studies indicates that progesterone is a risk factor for breast cancer and that alteration in progesterone/PR signaling pathways contributes to early stage human breast cancer progression. However, loss of PR expression in primary tumors is associated with a less differentiated more invasive phenotype and worse prognosis, suggesting that PR may limit later stages of tumor progression.

From the ranks of mammary progesterone mediators, RANKL takes the spotlight

Whether during the diestrus phase of the estrous cycle or with pregnancy onset, the mitogenic effects of progesterone are well-established in the murine mammary epithelium. Importantly, progesterone-induced mitogenicity is critical for mammary tumor promotion, providing one explanation for the increase in breast cancer-risk observed with prolonged progestin-based hormone therapy. At the cellular level, progesterone projects its mitogenic influence through an evolutionary conserved paracrine mechanism of action. In this regard, recent studies provide compelling support for receptor activator of NF-kB ligand (RANKL) as a key paracrine mediator of the progesterone mitogenic signal. Induction of RANKL is sufficient to elicit mammary ductal side-branching and alveologenesis, the very morphogenetic responses elicited by progesterone during pregnancy and at diestrus. Significantly, the proliferative and pro-survival signals triggered by RANKL are also required for progestin-promotion of mammary tumorigenesis, underscoring a dual role for RANKL in progesterone-dependent mammary morphogenesis and tumorigenesis. Recently, RANKL has been shown to be critical for progesterone-induced expansion of the mammary stem cell population (and its lineal descendents), thereby advancing our conceptual understanding not only of RANKL's involvement in normal mammary morphogenesis but also in breast cancer risk associated with sustained hormone exposure. Finally, these studies together suggest that chemotherapeutic intervention of RANKL signaling represents a feasible approach for the effective prevention and/or treatment of hormone-responsive breast cancers.

BRCA1 promoter methylation status does not predict response to tamoxifen in sporadic breast cancer patients

The purpose of this study is to investigate whether BRCA1 promoter methylation is associated with poorer outcome in sporadic breast cancer cases treated with tamoxifen. BRCA1 promoter methylation was determined by bisulfite pyrosequencing in two groups of sporadic breast cancer patients, systemically untreated (N = 497) and treated with adjuvant tamoxifen (N = 497). Associations of BRCA1 promoter methylation with clinopathological characteristics and the effect of BRCA1 promoter methylation on time to first recurrence (TTR) and overall survival (OS) were examined. No significant differences were observed between BRCA1 promoter methylation and clinopathological characteristics in untreated and tamoxifen-treated groups. Cut point analysis did not find any promising cut point for BRCA1 promoter methylation that would differentially influence TTR and OS in untreated and tamoxifen-treated group. Using the median (2.53 %) and an arbitrary value of 10 % as a cut point for methylation, we still found no significant effect of BRCA1 promoter methylation on TTR and OS in untreated and tamoxifen-treated group. Despite data suggesting that BRCA1 levels impact estrogen receptor response to tamoxifen, our results indicate that BRCA1 promoter methylation is not associated with poorer outcome in sporadic breast cancer cases treated with tamoxifen.

Active allies: hormones, stem cells and the niche in adult mammopoiesis

Adult stem cells are recruited in response to specific physiological demands to regenerate, repair or maintain essential cellular components of tissues, while preserving self-renewal capacity. Signals that activate adult stem cells are not simply cell autonomous and stem cells are part of a larger dynamic framework, the stem cell 'niche', which integrates systemic and local cues to sustain stem cell functionality. The mammary stem cell niche responds readily to hormonal stimuli, generating pertinent signals that activate stem cells, culminating in stem cell expansion and tissue growth. We review here current knowledge of the mammary stem cell niche with attention to the potent stimulation rendered by ovarian hormones, relevant cellular and molecular players, and the implication of a deregulated niche, for breast cancer risk.

Antiprogestins in breast cancer treatment: are we ready?

Breast cancer is the most frequently diagnosed cancer and the leading cause of cancer death in females worldwide. It is accepted that breast cancer is not a single disease, but instead constitutes a spectrum of tumor subtypes with distinct cellular origins, somatic changes, and etiologies. Molecular gene expression studies have divided breast cancer into several categories, i.e. basal-like, ErbB2 enriched, normal breast-like (adipose tissue gene signature), luminal subtype A, luminal subtype B, and claudin-low. Chances are that as our knowledge increases, each of these types will also be subclassified. More than 66% of breast carcinomas express estrogen receptor alpha (ERα) and respond to antiestrogen therapies. Most of these ER+ tumors also express progesterone receptors (PRs), the expression of which has been considered as a reliable marker of a functional ER. In this paper we will review the evidence suggesting that PRs are valid targets for breast cancer therapy. Experimental data suggest that both PR isoforms (A and B) have different roles in breast cancer cell growth, and antiprogestins have already been clinically used in patients who have failed to other therapies. We hypothesize that antiprogestin therapy may be suitable for patients with high levels of PR-A. This paper will go over the experimental evidence of our laboratory and others supporting the use of antiprogestins in selected breast cancer patients.

Downregulation of the tumor-suppressor miR-16 via progestin-mediated oncogenic signaling contributes to breast cancer development

Introduction

Experimental and clinical evidence points to a critical role of progesterone and the nuclear progesterone receptor (PR) in controlling mammary gland tumorigenesis. However, the molecular mechanisms of progesterone action in breast cancer still remain elusive. On the other hand, micro RNAs (miRNAs) are short ribonucleic acids which have also been found to play a pivotal role in cancer pathogenesis. The role of miRNA in progestin-induced breast cancer is poorly explored. In this study we explored progestin modulation of miRNA expression in mammary tumorigenesis.

Methods

We performed a genome-wide study to explore progestin-mediated regulation of miRNA expression in breast cancer. miR-16 expression was studied by RT-qPCR in cancer cell lines with silenced PR, signal transducer and activator of transcription 3 (Stat3) or c-Myc, treated or not with progestins. Breast cancer cells were transfected with the precursor of miR-16 and proliferation assays, Western blots or in vivo experiments were performed. Target genes of miR-16 were searched through a bioinformatical approach, and the study was focused on cyclin E. Reporter gene assays were performed to confirm that cyclin E 3'UTR is a direct target of miR-16.

Results

We found that nine miRNAs were upregulated and seven were downregulated by progestin in mammary tumor cells. miR-16, whose function as a tumor suppressor in leukemia has already been shown, was identified as one of the downregulated miRNAs in murine and human breast cancer cells. Progestin induced a decrease in miR-16 levels via the classical PR and through a hierarchical interplay between Stat3 and the oncogenic transcription factor c-Myc. A search for miR-16 targets showed that the CCNE1 gene, encoding the cell cycle regulator cyclin E, contains conserved putative miR-16 target sites in its mRNA 3' UTR region. We found that, similar to the molecular mechanism underlying progestin-modulated miR-16 expression, Stat3 and c-Myc participated in the induction of cyclin E expression by progestin. Moreover, overexpression of miR-16 abrogated the ability of progestin to induce cyclin E upregulation, revealing that cyclin E is a novel target of miR-16 in breast cancer. Overexpression of miR-16 also inhibited progestin-induced breast tumor growth in vitro and in vivo, demonstrating for the first time, a role for miR-16 as a tumor suppressor in mammary tumorigenesis. We also found that the ErbB ligand heregulin (HRG) downregulated the expression of miR-16, which then participates in the proliferative activity of HRG in breast tumor cells.

Conclusions

In this study, we reveal the first progestin-regulated miRNA expression profile and identify a novel role for miR-16 as a tumor suppressor in progestin- and growth factor-induced growth in breast cancer.

The chemopreventive effect of mifepristone on mammary tumorigenesis is associated with an anti-invasive and anti-inflammatory gene signature

Progesterone receptor (PR) antagonists are potent antitumor agents in carcinogen and progestin-dependent mammary tumorigenesis models through both PR- and non-PR-mediated mechanisms. The PR antagonist mifepristone/RU486 has been used primarily as an abortifacient possessing high affinity for both the PR and glucocorticoid receptors (GR). To determine whether mifepristone would be effective as a chemopreventive agent, we assessed its effect on progestin/7,12-dimethylbenz(a)anthracene (DMBA)-induced mammary carcinogenesis in wild-type (WT) and estrogen receptor-α-positive (ER(+)) transgenic mice expressing the dominant-negative Pax8PPARγ (Pax8) fusion protein. Mifepristone administered at a dose of 2.5 mg significantly delayed mammary tumorigenesis in WT, but not in Pax8 mice, whereas, a three-fold higher dose almost completely blocked tumorigenesis in both WT and Pax8 mice. The sensitivity of WT mice to 2.5 mg mifepristone correlated with an expression profile of 79 genes in tumors, 52 of which exhibited the opposite response in Pax8 mice, and corresponded primarily to the downregulation of genes associated with metabolism, inflammation, and invasion. These results suggest that the chemopreventive activity of mifepristone in WT mice correlates with a specific gene expression signature that is associated with multiple nuclear receptor signaling pathways.

Evolutionary pathways in BRCA1-associated breast tumors

BRCA1-associated breast tumors display loss of BRCA1 and frequent somatic mutations of PTEN and TP53. Here we describe the analysis of BRCA1, PTEN, and p53 at the single cell level in 55 BRCA1-associated breast tumors and computational methods to predict the relative temporal order of somatic events, on the basis of the frequency of cells with single or combined alterations. Although there is no obligatory order of events, we found that loss of PTEN is the most common first event and is associated with basal-like subtype, whereas in the majority of luminal tumors, mutation of TP53 occurs first and mutant PIK3CA is rarely detected. We also observed intratumor heterogeneity for the loss of wild-type BRCA1 and increased cell proliferation and centrosome amplification in the normal breast epithelium of BRCA1 mutation carriers. Our results have important implications for the design of chemopreventive and therapeutic interventions in this high-risk patient population.

Antitumor agents. 289. Design, synthesis, and anti-breast cancer activity in vivo of 4-amino-2H-benzo[h]chromen-2-one and 4-amino-7,8,9,10-tetrahydro-2H-benzo[h]chromen-2-one analogues with improved water solubility

Previously, we reported that 4-amino-2H-benzo[h]chromen-2-one (ABO) and 4-amino-7,8,9,10-tetrahydro-2H-benzo[h]chromen-2-one (ATBO) analogues, which were developed from the lead natural product neo-tanshinlactone, are potent cytotoxic agents. In order to improve on their water solubility, the diamino analogues and related salts were designed. All synthesized compounds were assayed for cytotoxicity, and selected compounds were evaluated for in vivo anti-mammary epithelial proliferation activity in wild-type mice and mice predisposed for mammary tumors due to Brca1/p53 mutations. The new derivatives 10, 16 (ABO), 22, and 27 (ATBO) were the most active analogues, with IC(50) values of 0.038-0.085 μM in the cytotoxicity assay. Analogue 10 showed around 50-fold improved water solubility compared with the prior lead ABO compound 4-[(4'-methoxyphenyl)amino]-2H-benzo[h]chromen-2-one (3). Compounds 3, 4, 10, and 22 significantly reduced overall numbers of mammary cells, as indicated by the reduction of mammary gland branching in mutant mice. A one-week treatment with 10 resulted in 80% reduction in BrdU-positive cells in the cancer prone mammary gland. These four compounds had differential effects on cellular proliferation and apoptosis in wild-type mouse and a mouse model of human breast cancers. Compound 10 merits further development as a promising anticancer clinical trial candidate.

Insulin-like growth factor 1 attenuates antiestrogen- and antiprogestin-induced apoptosis in ER+ breast cancer cells by MEK1 regulation of the BH3-only pro-apoptotic protein Bim

Introduction

In this pre-clinical in vitro study conducted in estrogen receptor positive (ER+) breast cancer cells, we have characterized the effects of insulin-like growth factor I (IGF-1) on the cytostatic and cytotoxic action of antiestrogen treatment when used as a single agent or in combination with the antiprogestin mifepristone (MIF). Our goal was to identify new molecular targets to improve the efficacy of hormonal therapy in breast cancer patients that have a poor response to hormonal therapy, in part, due to high circulating levels of unbound insulinIGF-1.

Methods

IGF-1-mediated effects on cytostasis and apoptotic cell death were determined with cell counts conducted in the presence and absence of trypan blue; enzyme-linked immunosorbent assays to determine the intracellular levels of cleaved cytokeratin 18, a marker of epithelial cancer cell apoptosis; and immunoblot analysis to determine the levels of cleaved poly-ADP ribose polymerase (PARP) and lamin A that result from caspase-dependent apoptosis. Cytotoxicity was further characterized by determination of the levels of reactive oxygen species (ROS) and the percent of mitochondrial membrane depolarization in cell populations treated with the different hormones in the presence and absence of IGF-1. Small molecule inhibitors of the dual-specificity protein kinase MEK1, MEK1 siRNA, Bim siRNA, and vectors overexpressing MEK1 wild type and mutant, dominant negative cDNA were used to identify key IGF-1 downstream prosurvival effectors.

Results

IGF-1, at physiologically relevant levels, blocked the cytotoxic action(s) of the antiestrogens 4-hydroxytamoxifen (4-OHT) and tamoxifen (TAM) when used as single agents or in combination with the antiprogestin MIF. The antiapoptotic action of IGF-1 was mediated primarily through the action of MEK1. MEK1 expression reduced the levels of ROS and mitochondrial membrane depolarization induced by the hormonal treatments via a mechanism that involved the phosphorylation and proteasomal turnover of the proapoptotic BH3-only Bcl-2 family member Bim. Importantly, small-molecule inhibitors of MEK1 circumvented the prosurvival action of IGF-1 by restoring Bim to levels that more effectively mediated apoptosis in ER⁺ breast cancer cells.

Conclusion

his study provides strong support for the use of MEK1 inhibitors in combination with hormonal therapy to effectively affect cytostasis and activate a Bim-dependent apoptotic pathway in ER⁺ breast cancer cells. We discuss that MEK1 blockade may be a particularly effective treatment for women with high circulating levels of IGF-1, which have been correlated to a poor prognosis.

Genome-wide progesterone receptor binding: cell type-specific and shared mechanisms in T47D breast cancer cells and primary leiomyoma cells

Background

Progesterone, via its nuclear receptor (PR), exerts an overall tumorigenic effect on both uterine fibroid (leiomyoma) and breast cancer tissues, whereas the antiprogestin RU486 inhibits growth of these tissues through an unknown mechanism. Here, we determined the interaction between common or cell-specific genome-wide binding sites of PR and mRNA expression in RU486-treated uterine leiomyoma and breast cancer cells.

Principal Findings

ChIP-sequencing revealed 31,457 and 7,034 PR-binding sites in breast cancer and uterine leiomyoma cells, respectively; 1,035 sites overlapped in both cell types. Based on the chromatin-PR interaction in both cell types, we statistically refined the consensus progesterone response element to G•ACA• • •TGT•C. We identified two striking differences between uterine leiomyoma and breast cancer cells. First, the cis-regulatory elements for HSF, TEF-1, and C/EBPα and β were statistically enriched at genomic RU486/PR-targets in uterine leiomyoma, whereas E2F, FOXO1, FOXA1, and FOXF sites were preferentially enriched in breast cancer cells. Second, 51.5% of RU486-regulated genes in breast cancer cells but only 6.6% of RU486-regulated genes in uterine leiomyoma cells contained a PR-binding site within 5 kb from their transcription start sites (TSSs), whereas 75.4% of RU486-regulated genes contained a PR-binding site farther than 50 kb from their TSSs in uterine leiomyoma cells. RU486 regulated only seven mRNAs in both cell types. Among these, adipophilin (PLIN2), a pro-differentiation gene, was induced via RU486 and PR via the same regulatory region in both cell types.

Conclusions

Our studies have identified molecular components in a RU486/PR-controlled gene network involved in the regulation of cell growth, cell migration, and extracellular matrix function. Tissue-specific and common patterns of genome-wide PR binding and gene regulation may determine the therapeutic effects of antiprogestins in uterine fibroids and breast cancer.

Genomic instability in breast and ovarian cancers: translation into clinical predictive biomarkers

Breast and ovarian cancer are among the most common malignancies diagnosed in women worldwide. Together, they account for the majority of cancer-related deaths in women. These cancer types share a number of features, including their association with hereditary cancer syndromes caused by heterozygous germline mutations in BRCA1 or BRCA2. BRCA-associated breast and ovarian cancers are hallmarked by genomic instability and high sensitivity to DNA double-strand break (DSB) inducing agents due to loss of error-free DSB repair via homologous recombination (HR). Recently, poly(ADP-ribose) polymerase inhibitors, a new class of drugs that selectively target HR-deficient tumor cells, have been shown to be highly active in BRCA-associated breast and ovarian cancers. This finding has renewed interest in hallmarks of HR deficiency and the use of other DSB-inducing agents, such as platinum salts or bifunctional alkylators, in breast and ovarian cancer patients. In this review we discuss the similarities between breast and ovarian cancer, the hallmarks of genomic instability in BRCA-mutated and BRCA-like breast and ovarian cancers, and the efforts to search for predictive markers of HR deficiency in order to individualize therapy in breast and ovarian cancer.

DHHC-7 and -21 are palmitoylacyltransferases for sex steroid receptors

Extranuclear sex steroid receptors require palmitoylation to traffic to the plasma membrane, where they activate signal transduction cascades. We identify DHHC-7 and -21 palmitoylacyltransferases as conserved enzymes for the three classes of sex steroid receptors.

Classical estrogen, progesterone, and androgen receptors (ERs, PRs, and ARs) localize outside the nucleus at the plasma membrane of target cells. From the membrane, the receptors signal to activate kinase cascades that are essential for the modulation of transcription and nongenomic functions in many target cells. ER, PR, and AR trafficking to the membrane requires receptor palmitoylation by palmitoylacyltransferase (PAT) protein(s). However, the identity of the steroid receptor PAT(s) is unknown. We identified the DHHC-7 and -21 proteins as conserved PATs for the sex steroid receptors. From DHHC-7 and -21 knockdown studies, the PATs are required for endogenous ER, PR, and AR palmitoylation, membrane trafficking, and rapid signal transduction in cancer cells. Thus the DHHC-7 and -21 proteins are novel targets to selectively inhibit membrane sex steroid receptor localization and function.

CDDO-methyl ester delays breast cancer development in BRCA1-mutated mice

The breast cancer-associated gene 1 (BRCA1) is the most frequently mutated tumor suppressor gene in familial breast cancers. Mutations in BRCA1 also predispose to other types of cancers, pointing to a fundamental role of this pathway in tumor suppression and emphasizing the need for effective chemoprevention in these high-risk patients. Because the methyl ester of the synthetic triterpenoid 2-cyano-3,12-dioxooleana-1,9(11)-dien-28-oic acid (CDDO-Me) is a potent chemopreventive agent, we tested its efficacy in a highly relevant mouse model of BRCA1-mutated breast cancer. Beginning at 12 weeks of age, Brca1(Co/Co); MMTV-Cre;p53(+/-) mice were fed powdered control diet or diet containing CDDO-Me (50 mg/kg diet). CDDO-Me significantly (P < 0.05) delayed tumor development in the Brca1-mutated mice by an average of 5.2 weeks. We also observed that levels of ErbB2, p-ErbB2, and cyclin D1 increased in a time-dependent manner in the mammary glands in Brca1-deficient mice, and CDDO-Me inhibited the constitutive phosphorylation of ErbB2 in tumor tissues from these mice. In BRCA1-deficient cell lines, the triterpenoids directly interacted with ErbB2, decreased constitutive phosphorylation of ErbB2, inhibited proliferation, and induced G(0)-G(1) arrest. These results suggest that CDDO-Me has the potential to prevent BRCA1-mutated breast cancer.

Choosing a mouse model: experimental biology in context--the utility and limitations of mouse models of breast cancer

Genetically engineered mice are critical experimental models for the study of breast cancer biology. Transgenic mice, employing strong mammary epithelial promoters to drive oncogenes, develop carcinomas with phenotypes corresponding to the molecular pathway activated. Gene-targeted (knockout) mice, in which tumor suppressors are deleted, develop mammary neoplasms with phenotypes primarily including patterns seen in spontaneous mouse mammary tumors, albeit at higher rates. Improved genetic engineering, using inducible gene expression, somatic gene transduction, conditional alleles, and crossbreeding for combined/compound genetic engineering yields precise molecular models with exquisite experimental control and phenotypes with comparative pathologic validity. Mammary gland transplantation technology adds a practical and validated method for assessing biologic behavior of selected mammary tissues. Overall, the many mouse models available are a rich resource for experimental biology with phenocopies of breast cancer subtypes, and a variety of practical advantages. The challenge is matching the model to the experimental question.

RINGs of good and evil: RING finger ubiquitin ligases at the crossroads of tumour suppression and oncogenesis

The ubiquitin-proteasome system has numerous crucial roles in physiology and pathophysiology. Fundamental to the specificity of this system are ubiquitin-protein ligases (E3s). Of these, the majority are RING finger and RING finger-related E3s. Many RING finger E3s have roles in processes that are central to the maintenance of genomic integrity and cellular homeostasis, such as the anaphase promoting complex/cyclosome (APC/C), the SKP1-cullin 1-F-box protein (SCF) E3s, MDM2, BRCA1, Fanconi anaemia proteins, CBL proteins, von Hippel-Lindau tumour suppressor (VHL) and SIAH proteins. As a result, many RING finger E3s are implicated in either the suppression or the progression of cancer. This Review summarizes current knowledge in this area.

BRCA1 counteracts progesterone action by ubiquitination leading to progesterone receptor degradation and epigenetic silencing of target promoters

Germ-line mutations in the BRCA1 gene increase the risk of breast cancer in women, but the precise mechanistic basis for this connection remains uncertain. One popular hypothesis to explain breast tissue specificity postulates a link between BRCA1 and the action of the ovarian hormones estrogen and progesterone. Given the relevance of progesterone for normal mammary development and breast cancer formation, we searched for a functional relationship between BRCA1 and progesterone receptor (PR) in the PR-positive breast cancer cell line T47D. Here, we report that BRCA1 inhibits the transcriptional activity of PR by at least 2 mechanisms involving the E3 ubiquitin ligase activity of BRCA1. First, BRCA1 has a direct effect on the cellular level of PR and, hence, on the extent of PR recruitment to target promoters through the promotion of its ligand-independent and -dependent degradation. Through in vitro and in vivo assays, we found that BRCA1/BARD1 may be the main E3 ubiquitin ligase responsible for ubiquitination and degradation of PR in the absence of hormone. Second, after hormone treatment of cells, the BRCA1/BARD1 complex is recruited via interaction with PR to the hormone-responsive regions of PR target genes, affecting local levels of monoubiquitinated histone H2A and contributing to epigenetic silencing of these promoters. The connections between BRCA1/BARD1 and PR activity suggested by our findings may help explain why host mutations in BRCA1 exert a tissue specificity in preferentially elevating the risk of breast cancer.

p38 and p42/44 MAPKs differentially regulate progesterone receptor A and B isoform stabilization

Progesterone receptor (PR) isoforms (PRA and PRB) are implicated in the progression of breast cancers frequently associated with imbalanced PRA/PRB expression ratio. Antiprogestins represent potential antitumorigenic agents for such hormone-dependent cancers. To investigate the mechanism(s) controlling PR isoforms degradation/stability in the context of agonist and antagonist ligands, we used endometrial and mammary cancer cells stably expressing PRA and/or PRB. We found that the antiprogestin RU486 inhibited the agonist-induced turnover of PR isoforms through active mechanism(s) involving distinct MAPK-dependent phosphorylations. p42/44 MAPK activity inhibited proteasome-mediated degradation of RU486-bound PRB but not PRA in both cell lines. Ligand-induced PRB turnover required neosynthesis of a mandatory down-regulating partner whose interaction/function is negatively controlled by p42/44 MAPK. Such regulation strongly influenced expression of various endogenous PRB target genes in a selective manner, supporting functional relevance of the mechanism. Interestingly, in contrast to PRB, PRA stability was specifically increased by MAPK kinase kinase 1-induced p38 MAPK activation. Selective inhibition of p42/p44 or p38 activity resulted in opposite variations of the PRA/PRB expression ratio. Moreover, MAPK-dependent PR isoforms stability was independent of PR serine-294 phosphorylation previously proposed as a major sensor of PR down-regulation. In sum, we demonstrate that MAPK-mediated cell signaling differentially controls PRA/PRB expression ratio at posttranslational level through ligand-sensitive processes. Imbalance in PRA/PRB ratio frequently associated with carcinogenesis might be a direct consequence of disorders in MAPK signaling that might switch cellular responses to hormonal stimuli and contribute towards pathogenesis.

Protective effects of prepubertal genistein exposure on mammary tumorigenesis are dependent on BRCA1 expression

This study investigated whether prepubertal dietary exposure to genistein reduces mammary tumorigenesis by upregulating Brca1 expression in mice. Heterozygous Brca1(+/-) mice and their wild-type (WT) littermates were fed control AIN93G diet or 500 ppm genistein-supplemented AIN93G diet from postnatal day (PND) 15 to PND30 and then switched to AIN93G diet. Prepubertal dietary exposure to genistein reduced 7,12-dimethylbenz(a)anthracene (DMBA)-induced mammary incidence (P = 0.029) and aggressiveness of the tumors (P < 0.001) in the WT mice and upregulated the expression of Brca1 in their mammary glands (P = 0.04). In contrast, prepubertal genistein diet neither significantly reduced mammary tumorigenesis or tumor aggressivity nor increased Brca1 mRNA expression in the Brca1(+/-) mice. These results may be related to the opposing effects of prepubertal genistein diet on the expression of Rankl and CK5/CK18 ratio (marker of luminal epithelial cell differentiation) in the mammary gland and estrogen receptor (ER-α) and progesterone receptor (PgR) protein levels in the mammary tumor: these all were reduced in the WT mice or increased in Brca1(+/-) mice. Both the WT and Brca1(+/-) mice exhibited reduced levels of amphiregulin, CK5, and CK18, delayed ductal elongation and a reduction in terminal end bud number in the normal mammary gland, and reduced HER-2 protein levels in the mammary tumors; however, these effects were not sufficient to significantly reduce mammary tumorigenesis in Brca1(+/-) mice. Our results show that upregulation of Brca1 may be required for prepubertal dietary genistein exposure to reduce later mammary tumorigenesis, perhaps because in the absence of this upregulation, mice do not exhibit genistein-induced downregulation of ER-α, PgR, and Rankl.

The BRCA1 ubiquitin ligase and homologous recombination repair

Impairment of homologous recombination (HR), a vital process employed during repair of DNA double strand breaks and stalled DNA replication, provides a valuable opportunity for the cell to become transformed. Once transformed, the impairment turns to be a target for therapy as exemplified by the synthetic lethal strategy such as poly (ADP-ribose) polymerase (PARP) inhibitor for BRCA1/2-defective breast and ovarian cancer. Hence, improving mechanistic understanding of HR has emerged as an urgent issue to address due to the high clinical demand. Ubiquitin modification plays a central role in HR and more than a few E3 ubiquitin ligases have been implicated in the process. However, the significance of the activity of one such key E3 ligase, BRCA1, has not yet been clarified and remains as a major obstacle in the field. Here, we review recent advances in our understanding of BRCA1 function in HR and discuss possible roles of the activity.

Mammary stem cells and their regulation by steroid hormones

Sustained exposure to estrogen and progesterone is a well-established risk factor for breast cancer. These hormones play a central role in the female reproductive cycle, in which they control morphogenesis of the mammary gland during puberty, ovulatory cycles and pregnancy. Mouse mammary stem cells (MaSCs) have recently been discovered to be highly responsive to female hormones, despite lacking expression of the estrogen and progesterone receptors. The inhibition of MaSCs by hormone receptor antagonists further suggests that these cells contribute to oncogenesis. Identification of paracrine mediators of hormone signaling to MaSCs may lead to the development of novel inhibitors that drive MaSCs into a more quiescent state. In this context, inhibition of the receptor activator of NF-κB/receptor activator of NF-κB ligand signaling pathway has profound implications for the prevention of breast cancer.

Progesterone receptor action: defining a role in breast cancer

The ovarian steroid hormones, estradiol and progesterone, and their nuclear receptors (estrogen receptor [ER] and progesterone receptor [PR]), are involved in breast cancer development. As ER-positive/PR-positive tumors progress, they are likely to become steroid hormone-resistant/independent, yet often retain expression of their steroid receptors. Notably, up to 40% of women with steroid receptor-positive tumors exhibit de novo resistance or eventually fail on estrogen- or ERα-blocking therapies (acquired resistance). Indeed, most of the research on this topic has centered on mechanisms of ER 'escape' from endocrine therapy and the design of better ER-blocking strategies; signaling pathways that mediate endocrine (i.e., anti-estrogen) resistance are also excellent therapeutic targets. However, serious consideration of PR isoforms as important drivers of early breast cancer progression and ER modulators is timely and significant. Indeed, progress has been hindered by ER-centric experimental approaches. This article will focus on defining a role for PR in breast cancer with hopes of providing a refreshing PR-focused perspective.

Mouse models of inherited cancer syndromes

Animal models of cancer have been instrumental in understanding the progression and therapy of hereditary cancer syndromes. The ability to alter the genome of an individual mouse cell in both constitutive and inducible approaches has led to many novel insights into their human counterparts. In this review, knockout mouse models of inherited human cancer syndromes are presented and insights from the study of these models are highlighted.